This invention relates to a metal detector.
Metal detectors are used in many industries for the detection of metal contaminants.
In the food and pharmaceutical industries metal contamination is a serious problem, and high performance is required in order to detect very small particles of all types of metal, and to effectively remove the metal from the product in order to safeguard the consumer.
Metal detectors are also used in other industries such as plastics, timber, and quarrying etc., for the protection of machinery. They are also used in airports and other establishments as a means of security for the detection of weapons.
In general metal detectors can be divided into two types:
1. The Enclosed Detector is an enclosure or box (usually metal) with a central hole or aperture through which the product to be tested is passed. The aperture can be round, square, rectangular or any other shape. This type of detector is very sensitive to the presence of metal inside the aperture. PA0 2. The Surface Detector is an enclosure with one external surface that is sensitive to the presence of metal. PA0 1. A number of coils of conductive material (copper wire) are wound on each side of the primary field generating coil. The coils on each side consist of a number of separate coils each being operative to be energised at the same frequency as the primary field coil, but of opposite polarity and graded in intensity (current) and phase. PA0 2. A number of individual coils are wound on each side of the primary field generating coil and all are short circuited and grounded. The coils can be in a single layer or multiple layer format and cover in a specific area of the aperture. PA0 3. Solid metal plates, tubes or screens, can be used, of such size, shape and position in relation to the primary field coil that the induced eddy currents generate an opposing field and by these means the required primary field loci can be tailored.
A conventional metal detector transducer comprises a balanced coil arrangement in the form of an AC bridge mounted at the centre of a metal box. A large distance between the coil and metal box is considered essential for high performance and minimising disturbance signals resulting from vibration effects and thermal or ageing effects. Vibration effects cause relative movement of the metal box and coil and the thermal or ageing effects can cause disturbance in the electrical balance of the coil system. Reduction in the distance from the coil to the metal box leads to exponential increase in these disturbance effects. It has therefore previously been thought undesirable to provide any metallic material within the vicinity of the aperture. Metal detector design principles have always been to keep metal as far away from the coils as possible.
These known metal detectors suffer from the drawback that they are sensitive to stationary metal, and even more so to moving metal, in areas outside of the detection zone. For an enclosed detector the detection zone is inside the aperture, and for a surface detector the detection zone is adjacent the sensitive surface.
Known metal detectors must therefore have an area outside of the detection zone that is free from metal. The area is known as the `metal free zone` or MFZ.
For high performance detectors this metal free zone can be very large, such that in order to fit a detector onto a conveyor, or a machine, a metal free zone is required which could be as much as five times the actual size of the metal detector itself. The size of the metal free zone may be calculated using the following equation: ##EQU1## where AH=Aperture Height or Diameter.
This requirement for a larger metal free area seriously effects where metal detectors can be used. It very often results in high installation costs due to the additional space required and in some instances rules out the possibility of installing a metal detector.
Also conventional metal detectors are disturbed by electrical loops in the supporting metal framework and great care has to be taken in the design of this framework to electrically isolate different sections to avoid this effect.
This `loop effect` can extend over far greater distances than the MFZ effect and can be as great as 10 times the aperture height or diameter.
The effect of an electrical loop which varies in its resistance owing to vibration effects or because of metal balls rotating in the ball race of a roller on a conveying system is to trigger the metal detector and cause such disturbance that the performance is seriously impaired.
Metal detectors in close proximity to each other operating at the same frequency will be disturbed by a `cross talk` effect which impairs sensitivity.
The present invention has been made from a consideration of these problems.